//
// Created by Jisam on 24/08/2024 8:01 PM.
// Solution of  P1208 [USACO1.3] 混合牛奶 Mixing Milk
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <cstdint>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <vector>
#include<climits>

using namespace std;

#define endl "\n"
#define PSI pair<string,int>
#define PII pair<int,int>
#define PDI pair<double,int>
#define PDD pair<double,double>
#define VVI vector<vector<int>>
#define VI vector<int>
#define VS vector<string>

#define PQLI priority_queue<int, vector<int>, less<int>>
#define PQGI priority_queue<int, vector<int>, greater<int>>
#define code_by_jisam ios::sync_with_stdio(false),cin.tie(nullptr),cout.tie(nullptr)
using namespace std;
using u32 = unsigned;
using i64 = long long;
using u64 = unsigned long long;
using i128 = __int128;
int dx[] = {-1, 1, 0, 0, 1, 1, -1, -1,};
int dy[] = {0, 0, -1, 1, 1, -1, -1, 1,};
void solution() {
    int n,m;
    cin >> n >> m;
    vector<PII> a(m);
    for(int i = 0 ; i < m ; i ++){
        cin >> a[i].first >> a[i].second;
    }
    sort(a.begin(),a.end(),[&](PII x ,PII b){
        return x.first != b.first ? x.first <b.first : x.second > b.second ;
    });
    int sum = 0;
    for(int i = 0 ; i < m ; i ++){
        if(n <= 0) break;
        sum += min(a[i].second,n) * a[i].first;
        n -= a[i].second;
    }
    cout << sum << endl;
}

int main() {
    code_by_jisam;
    int T = 1;
//    cin >> T;
    while (T--) {
        solution();
    }
    return 0;
}